captioner.rs had grown into two things: the (currently disabled)
Florence-2 captioner plus generic infrastructure the live tagger
depends on. Split the neutral parts out:
- download.rs: resilient curl downloader (resume, stall detection)
and NuGet package extraction
- onnx_runtime.rs: shared ONNX Runtime/DirectML DLL manifest,
provisioning, and ort init, with runtime_dir() as the single
definition of the DLL location (kept inside the caption model dir
so existing installs do not re-download)
Also dedupes the tagger-side copies of the DLL list and four
hardcoded caption-model path literals, and rewords tagger runtime
errors that wrongly told users to download the caption model.
